Radio communication system



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24, 1946. G. HOLLINGSWORTH EI'AL 2,413,183

RADIO COMMUNICATION SYSTEM Filed Feb. 10, 1945 M2 21,; Z; SUBSTITUTE FOR MISSING XR Kzideufl r 7 fw s e. Wi ma 7" W a 1 m 'jf g W 57. 53

SOURCE SIGNAL VOLTAGE Inventors: Guih ord LHoHingsvvor-th, Richard C.Jensen,

Their Attorney.

Patented Dec. 24, 1946 RADIO COMIVIUNICATIONv SYSTEM Guilford L. Hollingsworth, Schenectady, and Richard C. Jensen, Scotia, N. Y., assignors to General Electric Company, a corporation of New York Application February 10, 1943, Serial No. 475,352

5 Claims.

This invention relates to communication systems and more particularly to pulse generators for such systems.

It is an object of our invention to provide a pulse generator of improved stability and reliability.

It is another object of our invention to provide new and improved means for accurately and simply varying the width of the pulses produced in a pulse generator,

Still another object of our invention is to provide a new and improved pulse generating means in which the pulse width is substantially independent of variations in circuit voltages, temperature variations, and changes in the characteristics of the discharge devices with age.

Multivibrators are frequently used in pulse generating devices. Because of stray capacity effects, etc., the minimum uls width for a multivibrator is of the order of one microsecond. It is also an object of our invention to provide a new and improved pulse generator of the type employing mpable of accurately producing pulses of width less than one microsecond. The features of our invention which we believe to be novel are set forth with particularity in the appended claims. Our invention itself, both as to its organization and manner of operation, together with further objects andadvantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 is a circuit diagram of a radio communication system embodying the principles of our invention, Figs. 2 and 3 illustrate pulse shapes at various portions of the circuit of Fig. 1, and Fig. 4-. illustrates a modification applicable to the circuit of Fig. 1.

In the drawing there is illustrated a pulse generator including a multivibrator which is of the start-stop type. Since the output of the multivibrator is negative, it is followed by a clipperinverter stage II to produce substantially square pulses of positive polarity to drive the first stage of amplification, indicated by the numeral l2. The amplifier I2 is transformer coupled to a second amplifier stage M by means of a transformer [3. In order to obtain economy of anode power, both stages of amplification are operated with cutoff bias applied to the control electrodes of the discharge devices associated therewith so that the amplifier l2 draws anode current only during the period of the pulses. The output of'the second amplifier stage is applied to an oscillator l5 which may be of the magnetron type. The use of transformer coupling between the amplifier stages permits all discharge devices to be operated with grounded cathodes, thus making special, low capacity filament transformers unnecessary and allowing all filaments to be operated from a common source.

In order accurately to control the width of the pulses impressed on the amplifiers and also vto terminate the pulses or to decrease the time duration of the pulses to a shorter period o f tir ne than is possible in the output of the multivibrator, there is provided a feedback circuit betweenilfi output of the amplifier stage l4 and the control electrode circuit of the first amplification stage l2. The feedback circuit comprises a suitable -delay means H, which may be adjustable if desired, for causing a negative potential to be applied to the control electrodes of the electron discharge device 66 of the amplifier l2 at a predetermined time after the initiation of the pulse. When the feedback voltage appears at the first driver or amplifier, it cuts off the pulse thus making the pulse width equal to the delay time of the delay means.

Describing our invention in greater detail, the multivibrator l0 illustrated herein comprises a double triode discharge device 20 having anodes 2|, 22, control electrodes 23, 24 and cathodes 25, 26. The cathodes are connected together and to ground. In order to provide anode potential for the multivibrator there is provided a rectifier 28 which may comprise a transformer 29 having a primary winding 30 connected to a suitable source of alternating current and secondary windings 3|, 32, 33 and 34. The winding 3| may be utilized for supplying power to the heaters of the electron discharge devices used in the circuit. The high voltage winding 32 is connected to the anodes 35 of the full wave rectifying device 36. The secondary winding 33 provides the filament current for the filament 31 of the rectifying discharge device. The center tap of the winding 32 constitutes the negative terminal for the rectifier, which is grounded, and one side of the filament 31 constitutes the positive terminal of the rectifier. The output of the rectifier is suitably fil tered by filtering means comprising an inductance 38 and a plurality of condensers 39.

The anode 2| of the multivibrator is connected to a point of positive potential through the resistances 40 and 52, and the anode 22 is connected to a point of positive potential through an inductance 98 and series connected resistors 4| and 52.

The control electrode 23 of the left-hand por- 4- I Q a i .1

- negative.

tion of the discharge device 20 is connected to ground through the switch 42 and one of the resistors 43, the purpose of which is described later. The control electrode 24 of the right-hand portlon of the discharge device 2|] is connected to a source of negative potential, indicated by the numeral 45, through a suitable resistor 44.

The source of potential 45 is illustrated as comprising a half wave rectifier device 450, the filament of which is connected across the secondary winding 34 of the transformer 29 and one of the filaments of which is connected to the anode 35 of the rectifying device 36 so that the right-hand portion of the winding 32 is utilized as the alternating current source for the rectifier 45a. In series with that portion of the winding 32 and the rectifying device 45a is a voltage divider comprising series connected resistances 46, 41 and 48. In order to filter the direct current output of the rectifier there are provided suitable condensers 49, 50 and connected in parallel. One side of each condenser ds grounded, the other sides are connected by resistances 46 and 41 and resistor 49 i connected in shunt with condenser 5|. Inspection of the circuit of the rectifier indicates that the left-hand end of resistance 46 is at the most negative potential and the lower end of 48 is at the most positive potential. Therefore, a negative potential is applied to the electrode 24.

In order to cause operation of the device In in regular multivibrator fashion there is provided a condenser 53 connected between the anode 22 and the control electrode 23 and another condenser 54 connected between the control electrode 24 and the anode 2|.

With the circuit shown, the right-hand side of the discharge device 20 is normally non-conducting and the left-hand portion conducting because of the negative bias applied to control electrode 24. If for any reason there is a slight decrease in the potential on the control electrode 23, the flow of current through the left-hand portion of the discharge device 20 is decreased. Therefore, the anode potential, which was relatively low because of the discharge through the device, increases. Due to the coupling condenser 54, the potential of the control electrode 24 becomes less If the increase is sufllciently great the right-hand portion of the discharge device 20 begins to conduct, the anode potential suddenly decreases, and the coupling capacitor 53 causes the left-hand control electrode 23 to go negative beyond cutofi. At the same time the conduction through the right-hand portion reaches a maximum. This condition remains substantially constant for a time determined by the resistance and capacity in the multivibrator circuit. After expiration of that time the condensers 53' and 54 no longer maintain constant the potentials of the associated control electrodes because of current flow through the resistances connected thereto and the entire action tends to take place in the reverse direction. Inasmuch as there is a negative bias applied to the control electrode 24, operation halts after one pulse is produced unless a sufficiently negative potential again appears on the control electrode 23.

The arrangement described above is advantageous because relatively good accuracy of operation is achieved. It the left-hand portion is normally non-conductive a positive triggering potential on electrode 23 is required to initiate operation of the multivibrator and conduction in the right-hand portion takes place until the potential of control electrode 23 rises sufficiently to cause the left-hand portion of the device to become operative. Variations between different tubes, change of characteristics of various discharge devices with age, temperature and voltage variations, etc., cause variations in the required triggering potential, thus producing inaccuracy in the time of firing. However, if the input portion is normally conducting, the appearance of a small negative potential on the control electrode causes operation.

It is apparent from the above description that a negative pulse is produced and impressed on the load resistance 4|. Means is provided for varying the width of the output pulses. If both sides of the multivibrator circuit have substantially the same resistance and capacity, the on and oil times of the sides of the discharge device 20 will be relatively equal under free running conditions. Means is provided for varying the time constant of the left-hand side of the device by connecting the control electrode 23 to ground through one of a plurality of resistances 43, each of the resistances having a different value. The provision of a selector switch 42 enables choice to be made of the grid leak resistance connected to the control electrode. By varying the time constants of the left-hand side of the multivibrator, the opposite, variation is attained on the right-hand side. Therefore, the width of the negative pulses appearing on resistance 4| is dependent upon the position of the switch 42.

In order to cause operation of the multivibrator, there is provided a suitablesource of signal voltage, 55, as, for example, a source of square waves. The signal voltage may be impressed on the electrode 23 through a potential divider indicated by the numeral 56 and a coupling capacity or condenser 51.

The negative pulses produced by a multivibrator of the type illustrated and described herein normally have approximately the shape shown in Fig. 2a although proportions are greatly exaggerated for clearness of illustration. The inductance 99 is provided to cause a more nearly rectangularly shaped pulse to be formed. The inductance should be of such a value that it cooperates with the stray capacity of the circuit to give the equivalent of a parallel resonant circuit at relatively high frequencies. As a result, the output circuit will pass more high frequency components and give a more nearly rectangular pulse as indicated at b in Fig. 2.

In order to invert the pulses and render the pulses still more rectangular, the negative pulses are impressed on the inverting device through a coupling condenser 58. The inverter may comprise an electron discharge device 59 of the pentode type, the negative pulses bein applied to the control electrode 60 which is also connected to ground through a suitable grid leak 6|. The cathode and suppressor electrode are grounded. The anode 62 is connected to a suitable positive potential through an inductance 63 and a suitable resistor 63a. The negative pulses appearing on the control electrodes 60 cause a decrease in the amount of current flowing through the electron discharge device and therefore cause an increase of the anode potential. The discharge device 59 is preferably arranged so that the appearance of pulses on the control electrode drives the device beyond cut-ofi and thereby produces a substantially flat topped pulse. As a result the extreme tip of the pulse is clipped as indicated by the horizontal dash line in Fig. 3a. In the absence of the inductance 63 the resultant pulse is apportion of Fig. 3a below the dashed line, but, as

was the case with the inductance 99, presence of r the inductance 63 causes a more nearly rectangularly shaped pulse to be produced as shown in Fig. 3b. The positive pulses are coupled to the control electrodes 64 of the amplifier or driver 14 which may comprise a device 66 of a double tetrode type by means of a coupling capacitor 61 and resistors 68. The control electrode 64 are connected through the resistances 68 and the resistance 68a to a suitable negative tap on the voltage divider of the source of potential 45. With this arrangement the discharge device draws current only during the time a sufliciently positive pulse is applied.

Inasmuch as negative pulses appear in the output of the amplifier l2, means is provided for inverting the pulses before application to the final output stage l4. This means comprises, in the illustrative ,form of our invention, transformer I3 having a primary winding 69 and a secondary winding 14. One end of the primary 69 is conducted through the anode resistances 69a to the anodes of the electron discharge device 66 and the other end of the primary windin is connected to a suitable source of positive potential.

The last mentioned source may comprise a halfwave rectifier H and a transformer having a primary connected to a suitable source of alternating current and a. high voltage winding connected between ground and the anode. The filament winding is connected across the filament of the rectifier. With this arrangement the center tap of the filament transformer constitutes the positive terminal of the source. In order to filter the output of the rectifier there is provided a suitable condenser 13. The secondary transformer winding 14 is connected between the control electrode 15 of the discharge device 16 and the negative terminal of the source of potential 45.

The lower end of the winding 14 is connected to ground for radio frequency currents through the filter condenser 49. With these connections the control electrode 15 is biased beyond cutoff and therefore current flows only during the application of a sufficiently positive pulse.

In order to provide potential for the screen electrode of the discharge device 16 there is provided a voltage divider including resistances 80 and Bi connected across the output of the rectifier 10, that is, from ground to the midtap of the secondary windin of the filament transformer 12a. Radio frequency currents are bypassed to ground by means of a suitable condenser 82 connected between the screen electrode and ground. Anode potential for the discharge device 16 is obtained from a suitable source, indicated by the plus sign, through an inductance 83 and a resistor 84. If the oscillator or source of high frequency oscillations is of the magnetron type, as illustrated, a relatively high source of potential is provided as, for example, 15 kilovolts. The coil or winding 83 is provided for the same purpose as the previously described anode inductances. The resistance 84 is necessary to damp out the voltages developed by the inductance 83 and minimizes or prevents oscillations on the tails of the pulses. Therefore, substantially square pulses are applied to the oscillator.

Under the foregoing circuit conditions, the negative pulses appearing in the output of the amplifier stage 12 are inverted in the secondary of the transformer l3 and appear as positive pulses on the control electrode 15 of the discharge device 6 16 in the second amplifier. The negative pulses appearing in the output of the discharge device 16 are impressed on a utilization circuit such as an oscillator. For illustrative purposes, the utilization circuit is shown to include a magnetron l5 upon the cathode circuit of which pulses are impressed and which is connected to an antenna 86 to transmit therefrom rapidly repeated, short duration, pulses of intense micro-wave radiation.

Multivibrators are capable of operation from approximately two to one million pulses per second and therefore the pulses vary in width from one-half a second to one microsecond. The explanation of the narrow limit is possibly the presence of stray capacities. In order to enable the production of narrower pulses than possible with ordinary multivibrator operation and at the same time to insure accurate pulse width, there is provided apparatus utilizing the pulses from the output of the discharge device 18 for terminating the pulses applied to the first amplifier stage 12 at predetermined times.

The ancde-to-cathode voltage of the-discharge device 16 is applied through a condenser 89,to a voltage divider comprising series connected resistances 90, BI and 92. A predetermined portion of the voltage across the voltage divider is fed back to the control electrodes 64 through the resistor 93, a switch 94, and a suitable delay means I1, such as an artificial transmission line, and a coupling condenser 95.

The delay means I! may comprise a plurality of parallel-connected condensers, one side of each being grounded and the other sides being connected by inductances 91. The resistors 93 and 98 are connected across the line at each end thereof in order properly to terminate the transmission line and to prevent reflections. Inasmuch as negative pulses are fed back, the control electrodes 64 have applied thereto a negative potential which cuts off or neutralizes the pulses at times dependent upon the constants of the delay circuit, thus making the pulse width equal to the delay time of the line.

Means is provided for varying the time delay, the illustrative means comprising the switch 94 having a movable arm and a plurality of switch points, the points being connected to different combinations of capacity and inductance in the time delay circuit. It is undesirable, in the arrangement shown, to cut off more than one-half of each of the pulses because if a feedback pulse is initiated before the first half of the pulse has passed through the amplifying device l2, the pulse will be terminated so early in a pulse cycle that the delayed pulse is terminated before the inverted pulse from the clipper and inverter II has disappeared from the control electrodes 64 and therefore two pulses instead of one appear in each pulse period. In such a case the two pulses are usually of different widths. Therefore, the switch arms of switches 42 and 94 are preferably connected together for simultaneous change so that as the width of the multivibrator pulses are increased under the control of the switch 42 the amount of delay pulse fed back can be increased in like proportion. If desired, time delay circuits may be cascaded so that the shortened pulses can be impressed on a second delay circuit, and so on. This modification is particularly practicable if low gain amplifier stages are used. After the final narrowing operation, a high gain amplifier may be used beneficially.

In order to insure a steep rise on the leadin edges or the pulses applied to the amplifier l2,

the circuit of Fig. 1 may be modified, as shown in Fig. 4, by providing a series connected condenser 93 and resistance Hill between the control electrode 15 of discharge device I6 and a point between the resistances BB and the condenser 61. With this arrangement, the voltage from the secondary of transformer I3 is fed back to the amplifier l2. The condenser 99 should be of such capacity that it becomes fully charged at or before the pulse reaches its peak amplitude so that feed back occurs only during the rise of voltage. The value of the resistance I should be such as to enable the desired amplitude of voltage to be obtained. Thus, if the voltage fed back is too great, the amount of resistance should be decreased, and vice versa.

While we have shown and described a particular embodiment of our invention. it will be obvious 'to those skilled in the art that changes and modifications may be made without departing from our invention in its broader aspects, and we, theref0re, -aim in the appended claims to cover all such 'changes and modifications as fall within the true spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. In a radio communication system, means for producing a succession of discrete pulses of predetermined time duration, means for amplifying said pulses, mean for impressing said pulses on said amplifying means, said amplifying means having an output circuit in which the amplified pulses appear, means utilizing each of said amplified pulses for disabling said amplifying means at predetermined times less than the full time duration of said pulses, and means utilizing the voltage appearing in said output circuit at the initiation of each of said pulses forcausing the pulses impressed on said amplifier to have a steep rise on the leading edges thereof.

2. In a radio communication system, means for producing a succession of discrete pulse-s, means for amplifying said pulses, means for impressing said pulses on said amplifying means, said amplifying means having an output circuit in which the amplified pulses appear, means utilizing the voltage appearing in said output circuit at the initiation of each of said pulses for causing the pulses impressed on said amplifier to have a steep rise on the leading edges thereof, and delay means responsive to initiation of each pulse in said output circuit for disabling said amplifying means and terminating said output pulse prior to termination of the impressed pulse.

3. In a radio communication system, means for producing a succession of discrete pulses, means for amplifying said pulses, means for impressing said pulses on said amplifying means, said amplifying means having an output circuit in which the amplified pulses appear, regenerative means for feeding back to said impressing means during initiation of each of said pulses a voltage from said output circuit in such phase that there is produced a steep rise on the leading edge of each of said pulses, and delayed degenerative feedback means responsive to initiation of each pulse in said output circuit for disabling said amplifying means and terminating said output pulse prior to termination of the impressed pulse.

4. In a radio communication system, means for producing a succession of discrete pulses, means for amplifying said pulses including an electron discharge device having a control electrode, an anode, and a cathode, an input circuit between said control electrode and said cathode, and an output circuit between said anode and said cathode, means for impressing said pulses on said input circuit, a transformer coupled to said output circuit, and means including a capacitor for deriving from the secondary winding of said transformer and supplying to said input circuit during initiation of each of said pulses a voltage in such phase that there is produced a steep rise on the leading edge of each of said pulses.

5. An electric pulse generating system comprising means for producing a series of discrete pulses of predetermined time duration, amplifyin means for said pulses including input and output circuits, regenerative means coupling said input and output circuits for increasing the steepness of the leading edge of each of said amplified pulses, a transmission line section energized by said amplified pulses independently to measure a predetermined time interval after initiation of each of said pulses, said time interval being less than said predetermined time duration, and means connecting said time delay network periodically to disable said amplifying means thereby to terminate each of said amplified pulses at the end of said time interval.

GUILFORD L. HOLLINGSWORTH. RICHARD C. JENSEN. 

